To date there are no tarp systems that work well on the unique side dump trailers. Side dump trailers actually dump their load from either side of the trailer. This unique feature prevents the use of the typical side to side roll over tarp since this would limit dumping to only one side. No dumping could occur over the stored tarp because the tarp would be destroyed. The existing bow-sliding systems, which move the bows and tarp from front to rear, have cables on each side of the trailer, which would be damaged when the trailer dumps on either side. These bow systems are limited to only rear dump trucks. Also the bows need to slide on the rails for support, and many times the loads carried would prevent this. The bows would have to be stored in front of the trailer, which means the side rails would have to be extended. Far and away the biggest problem with all the bow-sliding covering systems is the fact that the stability of each bow relies on the cable, and the cable is also the point of propulsion. All existing bow-sliding systems work with a cable (or cables). The rear bow is fastened to the cable on each side of the trailer. All other bows have a hole in the bow end that the cable is threaded through. As the cable moves, the rear bow pulls the other bows toward the rear of the trailer by the tarp material. When the cable moves in the opposite direction the rear bow pushes the other bows to the front of the trailer. The cable must slide through the "bow end" of the other bows, but the cable must also be the stabilizing factor that keeps each bow standing upright as it moves to the front or to the rear of the trailer. In order to stabilize the bow (i.e., keep the bow from tilting forward or backward) parts of the bow "lean" on the cable, thereby binding on the cable as it attempts to slide through the bow end. All bow ends on the market today are one and one half to two inches wide, and most bows rise twelve to eighteen inches in the center, and the distance (length of the bow) averages ninety-six inches across. Therefore, you have a ninety-six inch long bow that rises twelve to eighteen inches in the center, and the stabilizing width is only one and one half to two inches. Any tilting of the bow places pressure on the cable in a one and one half to two inch area, and the average trailer will have thirteen to fifteen bows. Jamming occurs frequently in these systems.
A brief summary of the known prior art follows below. U.S. Pat. No. 3,481,645 (1969) to Stepp discloses a hollow track mounted on each side at the top of a dump truck. The top and bottom surfaces of the track support a set of wheels which are mounted on a T shaped carriage. The T shaped carriages support a plurality of bows which carry a tarp. Inside the hollow track is a chain drive mechanism which is powered to open and close the tarp cover over the truck.
U.S. Pat. No. 5,338,084 (1994) to Wardell discloses a flatbed trailer cover system having tracks on the outside top edges of the flatbed. Roller wheels support a base unit which supports bows and a tarp.
U.S. Pat. No. 2,757,042 (1956) to Schultz discloses a tarp support consisting of side mounted tracks which hold the ball end of a rod (5). An opposite end of the rod is tied to the bottom edge of a tarp.
U.S. Pat. No. 5,112,097 (1992) to Turner, Jr. discloses side mounted tracks which support a wheeled trolley which carries the back end of a tarp.
The closest known prior art is Stepp '645. Stepp teaches a sturdy track mounted on each side of the truck to support a trolley which in turn supports bows and a tarp. Stepp uses a chain and pulley system located inside the track to propel the trolleys. The system (nor any system) does not teach extending the tracks beyond the body of the truck to allow full use of all of the open top area in the dump unit. The chains, track and pulleys are prone to collecting dirt since they require lubrication. A dent in the track could easily jam the chain. The system was never commercialized.
The present invention improves the prior art devices by storing the bows in the open mode in an extension rack which can extend forward of the dump unit in rear and side dumpers or rear of the dump unit in side dumpers. Second, a sturdy and simple track and trolley system allows the bows to extend lower down the sides of the dump unit to avoid damage from tractor loading from the side. Third, the simple cable drive system prevents jamming. Fourth, the trolley system, in combination with modular bows, allows bows to be made higher to permit higher payloads in the dump unit.